1. Field of the Invention
The present invention relates to a clutch operational control apparatus used for automatically controlling the connection and disconnection of power transmitting paths between an engine and road wheels. It particularly relates to an operational control apparatus for a clutch which is provided in the power transmitting path of an vehicular automatic transmission.
2. Related Prior Arts
When a driving range shift position is selected in an vehicular automatic transmission, the clutch is engaged to transmit the engine power to the wheels. When the accelerator is fully closed (or an accelerator pedal is fully released) and the vehicle brake is released, the clutch is still engaged in the driving range to transmit some degree of the engine power (so-called the creep power) to the wheels, thereby allowing the vehicle to move slowly (to allow a creeping movement). The creeping movement enables a driver to easily maneuver the vehicle when driving the vehicle to park by a curb or into a garage.
However, when the vehicle brake is applied, the creeping force should be made as small as possible to avoid an unfavorable vibration of the vehicle and to improve the fuel consumption of the engine. From these view points, a control apparatus to control the creeping force based on the braking operation (the degree of the depression in the brake pedal) is disclosed in Japanese Laid-open Patent Publication No.62-216842. In the control unit, an engaging force of a clutch is so controlled as to correspond with a reference value which is set based on the braking operation. Accordingly, when the brake pedal is released, the engaging force of the clutch is increased, thereby transmitting a normal (or strong) creep force through the clutch. When the brake pedal is slightly depressed, the engaging force is decreased, thereby transmitting a weak creep force. Further, when the brake pedal is strongly depressed, the clutch is released to make the engaging force zero, thereby transmitting no creep force. In a control apparatus disclosed in Japanese Laid-open Patent Publication 60-245838, when a brake is applied, the engine idling speed is decreased to a specified low speed to decrease a creep force. The specified low speed corresponds to the engine idling speed which is set at a neutral range.
However, in the case of the control apparatus disclosed in Japanese Laid-open Patent Publication No.62-216842, the transmitted force to the wheels varies as a coefficient of friction in the clutch lining varies even if the clutch engaging force is kept unchanged. As a result, it is difficult to obtain a stable creep force. Particularly, it is difficult to obtain a low and stable creep force. In order to obtain a low and stable creep force, it is quite necessary to control the clutch engaging force accurately so as to coincide with a reference value. However, the outputs from sensors (such as pressure sensors) will always include a certain amount of errors. These errors may have a significant influence on the control of the clutch engaging force particularly when the required creep force is low. Accordingly, it is difficult to control the clutch engaging force accurately to obtain a low and stable creep force.
If the clutch engaging force is decreased to zero when the brake pedal is depressed, a clutch disk is fully moved to a disengaging position by a biassing force of a return spring in the clutch. Then, when the brake pedal is released and the accelerator pedal is depressed to accelerate the vehicle, the clutch disk must move from the disengaging position to an engaging position. The time required to move the clutch disk from the disengaging position to the engaging position may produce a lag before the clutch actually begins to engage. Since the engine speed can be increased rapidly under a disengaged condition of the clutch, a jerky engagement of the clutch may happen.
In the control apparatus disclosed in Japanese Laid-open Patent Publication 60-245838, the clutch engaging force is controlled using the engine idling speed as a reference value. However, since the engine idling speed does not always correspond with the engine load, it is difficult to control the creep force accurately. Otherwise, a complicated control is requires.